1. Field of the Invention
The present invention relates to an LED (light-emitting diode) driver and, more specifically, to an LED driver controller that supports multiple strings of LEDs.
2. Description of the Related Arts
LEDs are being adopted in a wide variety of electronics applications, for example, architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices including personal computers and high definition TVs, flashlights, etc. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety.
LEDs are current-driven devices, and thus regulating the current through the LEDs is an important control technique. To drive a large array of LEDs from a direct current (DC) voltage source, DC-DC switching power converters such as a boost or buck-boost power converters are often used to supply the top rail voltage for several strings of LEDs. In Liquid Crystal Display (LCD) applications using LED backlights, it is often necessary for a controller to control several strings of LEDs with independent current settings for each string. The controller can then independently control the brightness of different sections of the LCD. Furthermore, the controller can turn different parts of the LCD on or off in a timed manner.
FIG. 1 illustrates a conventional LED driver 100. LED driver 100 includes a boost DC-DC power converter 101 coupled between DC input voltage Vin and multiple strings of LEDs 102 (i.e., LED channels). The output Vboost of boost converter 101 is coupled to the anode of the first LED in each LED string 102. The cathode of the last LED in each string 102 is coupled to channel controller 115 for controlling the current in the string 102.
Each channel controller 115 comprises a PWM transistor 103 coupled in series with a Linear Drop Out regulator (LDO) 104. LDO 104 ensures that the peak current in LED string 102 is regulated to a fixed level. The peak current level is normally set to the same value as indicated by signal 108 for all LED channels by LDO reference controller 107. PWM transistor 103 controls the brightness of LED string 102 according to a Pulse Width Modulated (PWM) duty cycle. The brightness is set independently for each LED channel by luminance control signals 111 from luminance controller 109 that adjusts the PWM duty cycle according to the set brightness.
In this conventional configuration, power is dissipated in LDOs 104 to regulate the peak current. LEDs are current controlled devices, meaning that the luminous flux generated from them is primarily a function of the current applied through them. Thus, LDOs 104 ensure that the brightness of each LED channel will be the same because the peak current through each LED channel is the same. LDOs 104 also provide a native power supply rejection that reduces the impact of the boost voltage ripple from Vboost on the luminance of LED strings 102. In each LED channel, LDO 104 dissipates power proportional to the product of the current through LED string 102, the PWM duty cycle, and the voltage drop across LDO 104.
Due to manufacturing differences between the LEDs, the voltage drop across each LED string 102 necessary to maintain a specified current level varies considerably. To compensate for the different forward voltages LED strings 102, different voltage drops are seen across each LDO 104. The VI curve of FIG. 2 illustrates the exponential relationships between voltage and current for two different LEDs (LED1 and LED2). Assuming, for example, that LDO reference controller 107 sets the peak current in each LED channel to 40 mA, LED1 must operate at a forward voltage drop of about 3.06 volts, while LED2 must operate at a forward voltage drop of about 3.26 volts. Thus, there is a difference between the forward voltage drops of LED1 and LED2 of about 0.2 volts. Assuming there are 10 LEDs having the characteristics of LED1 in a first LED string 102, there is a 30.6 V drop across the string 102. Assuming there are 10 LEDs having the characteristics of LED2 in a second LED string 102, there is a 32.6 V drop across the second LED string 102. This difference of 2 volts will therefore be dissipated by the LDO that is driving the second string such that both strings operate at the same peak current of 40 mA. The total power dissipated is 80 mW. When these power losses are extended over many LED channels, they can become prohibitive.
An alternative approach to LED driver 100 of FIG. 1 uses current mirrors that each drive one LED string, as described, for example, in U.S. Pat. No. 6,538,394 issued to Volk et al. However, this current mirror approach suffers from low power efficiency. When the forward voltages of the LEDs differ, Vboost applied to the parallel-connected LED strings has to be higher than the forward voltage drop across the LED string with the highest combined forward voltage ΣVF. There is a voltage difference (Vboost−ΣVF) in the LED strings with a combined forward voltage lower than the highest, and this voltage difference is applied across each current mirror. Since the power dissipated by the current mirrors does not contribute to lighting, the overall efficiency is low, especially when the difference in the combined forward voltage between the LED strings is large.
A third conventional approach operates by turning on each of the multiple LED strings sequentially, as described in U.S. Pat. No. 6,618,031 issued to Bohn, et al. However, this approach requires fast dynamic response from the LED driver, and thus forces the power converter to operate in deep discontinuous mode (DCM), under which power conversion efficiency is low.
Yet another approach is to operate in a full digital switch mode without any LDO, as described in U.S. patent application Ser. No. 12/164,909 by Yuhui Chen, et al. filed on Jun. 30, 2008. In this approach, a PWM controller automatically adjusts the PWM duty cycle for each string to compensate for the varying peak currents. However, this approach results in a wide variation of LED currents between LED strings and leads to LED component stress and reliability control issues. Additionally, this conventional solution does not provide any native power supply rejection to the boost controller ripple, and thus complicates ADC and digital signal processing requirements.